Mars Water, Escher-style Puzzle

Polygonal cracking point to water freeze-thaw cycles, as seen from orbit. Whether this evidence supports the hypothesis depends on perspective, like the famous Escher drawings of lizards moving from two to three dimensions. When Mars is viewed from orbit in two-dimensions relative to the surface texture, scientists can explain features. Credit: NASA/JPL/Cornell

NASA’s Spirit and Opportunity have been exploring Mars about three times as long as originally scheduled. The more they look, the more evidence of past liquid water on Mars these robots discover. Team members reported the new findings at a news briefing today.

New findings raise the possibility Opportunity’s work area was soaked long ago, before it dried and eroded into a wide plain. There are also signs some rocks may have gotten wet again, after an impact excavated a stadium-size crater in the plain.

Evidence of this exciting possibility has been identified in a flat rock dubbed "Escher" and some neighboring rocks near the bottom of the crater. These plate-like rocks bear networks of cracks dividing the surface into patterns of polygons, somewhat similar in appearance to cracked mud after the water has dried up here on Earth.

Alternative histories, such as fracturing by the force of the crater-causing impact, or the final desiccation of the original wet environment that formed the rocks, might also explain the polygonal cracks. Rover scientists hope a lumpy boulder nicknamed "Wopmay," Opportunity’s next target for inspection, may help narrow the list of possible explanations.

"When we saw these polygonal crack patterns, right away we thought of a secondary water event significantly later than the episode that created the rocks," said Dr. John Grotzinger. He is a rover-team geologist from the Massachusetts Institute of Technology, Cambridge, Mass. Finding geological evidence about watery periods in Mars’ past is the rover project’s main goal, because such persistently wet environments may have been hospitable to life.

Viking picture of frost. Credit: NASA/JPL/Cornell

"Did these cracks form after the crater was created? We don’t really know yet," Grotzinger (banner image) said.

Sinuous channel as outflow. Credit: NASA

If they did, one possible source of moisture could be accumulations of frost partially melting during climate changes, as Mars wobbled on its axis of rotation, in cycles of tens of thousands of years. According to Grotzinger, another possibility could be the melting of underground ice or release of underground water in large enough quantity to pool a little lake within the crater.

One type of evidence Wopmay could add to the case for wet conditions after the crater formed would be a crust of water- soluble minerals. After examining that rock, the rover team’s plans for Opportunity are to get a close look at a tall stack of layers nicknamed "Burns Cliff" from the base of the cliff. The rover will then climb out of the crater and head south to the spacecraft’s original heat shield and nearby rugged terrain, where deeper rock layers may be exposed.

Gullies at crater rim. Credit: NASA

Halfway around Mars, Spirit is climbing higher into the "Columbia Hills." Spirit drove more than three kilometers (approximately two miles) across a plain to reach them. After finding bedrock that had been extensively altered by water, scientists used the rover to look for relatively unchanged rock as a comparison for understanding the area’s full range of environmental changes. Instead, even the freshest-looking rocks examined by Spirit in the Columbia Hills have shown signs of pervasive water alteration.

"We haven’t seen a single unaltered volcanic rock, since we crossed the boundary from the plains into the hills, and I’m beginning to suspect we never will," said Dr. Steve Squyres of Cornell University, Ithaca, N.Y., principal investigator for the science payload on both rovers. "All the rocks in the hills have been altered significantly by water. We’re having a wonderful time trying to work out exactly what happened here," he added.

Water ice and dry ice at poles with seasonal changes. Credit: NASA

More clues to deciphering the environmental history of the hills could lie in layered rock outcrops further upslope, Spirit’s next targets. "Just as we worked our way deeper into the Endurance crater with Opportunity, we’ll work our way higher and higher into the hills with Spirit, looking at layered rocks and constructing a plausible geologic history," Squyres said.

Jim Erickson, rover project manager at JPL, said, "Both Spirit and Opportunity have only minor problems, and there is really no way of knowing how much longer they will keep operating. However we are optimistic about their conditions, and we have just been given a new lease on life for them, a six-month extended mission that began Oct. 1. The solar power situation is better than expected, but these machines are already well past their design life. While they’re healthy, we’ll keep them working as hard as possible."